The embodiments described herein relate generally to a position indication system, and more specifically, to a vibration tolerant bearing mechanism for use in a position indication system.
Closed loop control of a linear actuator typically requires an accurate sensed position feedback signal. Linear actuators are included in many residential and commercial applications. For example, positions of valves included within a steam turbine may be adjusted using linear actuators. Furthermore, a position of the valve may be measured using a position indication device, for example, a linear variable differential transformer (LVDT). A steam valve position indication system may utilize a stud coupled to the steam valve in combination with a rod end bearing arrangement to secure a linear transducer rod of the LVDT to the steam valve. The rod end bearing arrangement may include a bearing ball and a bearing housing.
Significant vibration levels encountered on large steam turbine valves may cause severe wear to the rod end bearing arrangement, for example, between the bearing ball and the bearing housing. The severity of the wear may allow the rod end bearing housing to disconnect from the bearing ball. The degrees of freedom of the rod end bearing arrangement also may allow for significant rotational vibration about the axis of the linear transducer rod, which contributes to increased wear. This wear may occur on both metal-on-metal greased rod end bearing arrangements and non-greased rod end bearing arrangements. Furthermore, a bearing ball liner, for example, a Teflon® weave liner between the bearing ball and the bearing housing, does not prevent such wear. Teflon® is a registered trademark of E. I. du Pont de Nemours and Company. Moreover, the liner may become deformed due to excessive vibration, resulting in excessive clearance between the bearing ball and the bearing housing.
Wear to the rod end bearing arrangement may result in insecure coupling of the rod end bearing arrangement and the stud. This allows relative motion to occur between the metal stud and metal rod end bearing housing. The relative motion may result in fretting wear of the stud, and ultimately, to failure of the stud. The relative motion may also result in a noisy feedback signal from LVDT to the control system.
To increase the useful life of the position indication system, the rod end bearing arrangement may be greased and/or materials used to form the bearing ball and the bearing housing may be varied. For example, elastomeric rod end bearing arrangements are commercially available, however, characteristics of a typical elastomeric rod end bearing arrangement are not adjustable based on a preload compression level.